Just want to let you know that there is a bundle for this. I am using GregwarCaptchaBundleavailable here that you can literally install in 2 minutes.
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MickJan 22 '13 at 8:34

Thanks @Patt! I installed yet and it's true, it's very easy to install, but at this moment I prefer reCaptcha from Google. Do you know if exists something as beforeLoginHandler or event or listener?
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SantiJan 22 '13 at 9:24